The invention relates to a method of making a borided dispenser cathode comprising a high melting temperature base material in which the emissive material is present in the form of a metal oxide, which metal oxide is reduced continuously during operation of the cathode and the metal diffuses to the surface in atomic form and forms there a monoatomic film. Such cathodes are used, for example, in magnetrons, transmitting tubes, X-ray tubes and klystrons.
The film forms a dipole surface layer as a result of which the work function is reduced below that of pure emitter material. Examples of such film cathodes are the thoriated carburized tungsten cathode (Th-[W].sub.c) and the carburized lanthaniated molybdenum cathode (La-[Mo].sub.c). Similar cathodes with other rare earth metals and with alkaline earth metals as emitters are also known. An improvement of the emissive properties is obtained by the carburization. Carburization of a thoriated-tungsten cathode is carried out, for example, in an organic vapour (for example, an H.sub.2 -benzene mixture) at 1,600.degree. to 2,000.degree. C. The activating process in such a carburized cathode is less critical, the life of the cathode is extended, and higher emission current densities during continuous operation of the cathode are achieved. Such cathodes are also less sensitive to ion bombardment and the evaporation of the emitter material is smaller than in a non-carburized cathode.
A method of making a borided dispenser cathode is known from Izvestiya Akademii Nauk S.S.S.R., Neorganischeskie Materialy, Vol. 15, No. 1, pp. 64-67, January, 1979. The replacement by a boride (WB, W.sub.2 B) of the carbide layer formed during the carburization improves the emission properties of thoriated tungsten. In the method described in this article, boron is provided in a thoriated tungsten wire by roasting it in a powder mixture which comprises boron. It is also possible to provide boron by brushing a boron carbide suspension on the cathode and then heating it. Making borided cathodes in a powder mixture or by means of a suspension requires a number of extra treatments in the production process.